m707 DLL.java

class MyLinkedList {
    private class Node {
        public int val;
        public Node next;
        public Node prev;

        public Node(int val, Node next, Node prev) {
            this.val = val;
            this.next = next;
            this.prev = prev;
        }
        public Node(int val, Node prev) {
            this(val, null, prev);
        }
        public Node(int val) {
            this(val, null);
        }
    }
    
    public Node head;
    public Node tail;
    public int size;

    public MyLinkedList() {
        this.size = 0;
    }
    
    public int get(int index) {
        if (head == null || index >= size || index < 0) {
            return -1;
        }

        Node current;
        if (index > size / 2) {
            current = tail;
            for (int i = size - 1; i > index; i--) {
                current = current.prev;
            }
        } else {
            current = head;
            for (int i = 0; i < index; i++) {
                current = current.next;
            }
        }

        return current.val;
    }
    
    public void addAtHead(int val) {
        if (head == null) {
            head = new Node(val);
            tail = head;
        } else {
            Node newHead = new Node(val);
            newHead.next = head;
            head.prev = newHead;
            head = newHead;
        }
        size++;
    }
    
    public void addAtTail(int val) {
        if (tail == null) {
            head = new Node(val);
            tail = head;
        } else {
            Node newTail = new Node(val);
            newTail.prev = tail;
            tail.next = newTail;
            tail = newTail;
        }
        size++;
    }
    
    public void addAtIndex(int index, int val) {
        if (index > size || index < 0) {
            return;
        }

        if (index == 0) {
            addAtHead(val);
            return;
        }

        if (index == size) {
            addAtTail(val);
            return;
        }

        Node current;
        if (index > size / 2) {
            current = tail;

            for (int i = size - 1; i > index - 1; i--) {
                current = current.prev;
            }
        } else {
            current = head;

            for (int i = 0; i < index - 1; i++) {
                current = current.next;
            }
        }

        Node newNode = new Node(val, current.next, current);
        current.next.prev = newNode;
        current.next = newNode;

        size++;
    }
    
    public void deleteAtIndex(int index) {
        if (index >= size || index < 0) {
            return;
        }

        if (index == 0) {
            if (size == 1) {
                head = null;
                tail = null;
                size--;
                return;
            }

            head = head.next;
            head.prev = null;
            size--;
            return;
        }

        if (index == size - 1) {
            tail = tail.prev;
            tail.next = null;
            size--;
            return;
        }

        Node current;
        if (index > size / 2) {
            current = tail;

            for (int i = size - 1; i > index - 1; i--) {
                current = current.prev;
            }

            current.next = current.next.next;
            if (current.next != null) {
                current.next.prev = current;
            }
        } else {
            current = head;

            for (int i = 0; i < index - 1; i++) {
                current = current.next;
            }

            current.next = current.next.next;
            current.next.prev = current;
        }

        size--;
    }
}

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * MyLinkedList obj = new MyLinkedList();
 * int param_1 = obj.get(index);
 * obj.addAtHead(val);
 * obj.addAtTail(val);
 * obj.addAtIndex(index,val);
 * obj.deleteAtIndex(index);
 */